The present invention relates to the production of polyesters having a high molecular weight and high chemical purity.
Heretofore, polyesters have been produced by a variety of processes, many of which have failed to achieve a high molecular weight with high purity composition. Specifically, the use of the conventional melt process requires relatively high levels of catalysts (e.g., between 300-650 ppm of SbO.sub.3 or greater than 10 ppm of Ti), to promote the polycondensation reaction within acceptable reaction times. The polyesters produced by the melt process usually contain more than 20 percent of the end groups having a carboxyl end group. Relatively high levels of catalyst and carboxyl end group content in polyethylene terephthalate produced by a conventional method contribute to chemical instability during processing and use. Therefore, stabilizers are usually added to improve the stability. For example, organic phosphorus is added to complex the catalyst and carboxiimide or epoxide is added to reduce the carboxyl end group content, the amounts of stabilizers required being proportional to the levels of catalyst and carboxyl end group contents in the polyester. Often, the additives are either not suitable for the particular commercial application, or are subject to government regulatory standards which inhibit their use. Even after the addition of the stabilizers, certain deleterious polymerization byproducts such as acetaldehydes present in the polyester reduce its purity, limiting the commercial applications of the polyester. The need for a high purity polyester composition presently exists in many commercial and industrial applications.
The production of polyesters by a melt process has been the conventional production method. However, the production of a polyester by conventional production methods requires the high levels of catalyst and yields significant carboxyl end group content, which is often a detriment to certain commercial and industrial applications mentioned above.